Solid-state laser device, solid-state laser system, and laser device for exposure device

ABSTRACT

A solid state laser device includes a seed laser that outputs continuous wave laser seed light, a light intensity changeable unit that changes a light intensity thereof and outputs seed pulse light, a CW excitation laser that outputs continuous wave excitation light, an amplifier that amplifies the seed pulse light and outputs amplified light based on an amplification gain increased by the excitation light, a wavelength conversion unit that converts a wavelength of the amplified light and outputs harmonic light, and a light intensity control unit that allows the light intensity changeable unit to output the seed pulse light after a certain time elapsed from an input of an external trigger signal each time the signal is input and output suppression light that suppresses an increase of the amplification gain in a period after an output of the seed pulse light until an input of a next external trigger signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of InternationalApplication No. PCT/JP2017/010674 filed on Mar. 16, 2017 claimingpriority to International Application No. PCT/JP2016/061358 filed onApr. 7, 2016. Each of the above applications is incorporated herein byreference in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a solid-state laser device, asolid-state laser system, and a laser device for an exposure device.

2. Related Art

Along with development of micronizing and high integration ofsemiconductor integrated circuits, an improvement in resolution isrequired in semiconductor exposure devices. A semiconductor exposuredevice will be simply referred to as an “exposure device” in thefollowing description. Accordingly, a wavelength of light output from anexposure light source is getting shortened. As an exposure light source,a gas laser device is used, instead of a conventional mercury lamp. Atpresent, as for laser devices for exposure, a KrF excimer laser deviceconfigured to output ultraviolet light having a wavelength of 248 nm,and an ArF excimer laser device configured to output ultraviolet lighthaving a wavelength of 193.4 nm are used.

Currently, as an exposure technology, immersion exposure is put intopractice. In the immersion exposure, a space between a projection lenson the exposure device side and a wafer is filled with liquid, wherebythe refractive index of the space is changed. Thereby, an apparentwavelength of the light source for exposure is shortened. In the casewhere immersion exposure is performed with use of an ArF excimer laserdevice as a light source for exposure, a wafer is irradiated withultraviolet light having a wavelength of 134 nm in the water. Thistechnology is called ArF immersion exposure. ArF immersion exposure isalso referred to as ArF immersion lithography.

The spectral linewidth in natural oscillation in KrF and ArF excimerlaser devices is wide, approximately ranging from 350 pm to 400 pm. Thiscauses chromatic aberration of laser light (ultraviolet light) reducedand projected on the wafer by the projection lens on the exposure deviceside. Thereby, the resolution is lowered. As such, it is necessary tonarrow the spectral linewidth of laser light output from a gas laserdevice to a degree in which chromatic aberration can be disregarded.Accordingly, a line narrowing module having a line narrowing element isprovided to the laser resonator of a gas laser device. By the linenarrowing module, narrowing of the spectral linewidth is realized. Theline narrowing element may be an etalon, a grating, or the like. A laserdevice in which the spectral linewidth is narrowed as described above isreferred to as a line narrowed laser device.

Meanwhile, as a laser device for an exposure device, there is aconfiguration including a master oscillator (MO) and a power oscillator(PO). In such a laser device for an exposure device, excimer laserdevices are used for the MO and the PO. However, from the viewpoint ofenergy saving, a laser device for an exposure device to which asolid-state laser device is applied is being developed. A solid-statelaser device is configured to include a semiconductor laser, a nonlinearcrystal, and the like. Such a solid-state laser device is applicable notonly to a laser device for an exposure device, but also to a laserdevice for processing and the like.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Laid-Open No.    2014-053627-   Patent Literature 2: Japanese Patent Application Laid-Open No.    2011-192831-   Patent Literature 3: Japanese Patent Application Laid-Open No.    11-285872-   Patent Literature 4: Japanese Patent Application Laid-Open No.    2009-229715-   Patent Literature 5: Japanese Patent Application Laid-Open No.    2012-199425

SUMMARY

A solid state laser device, according to one aspect of the presentdisclosure, may include a seed laser, a light intensity changeable unit,a CW excitation laser, an amplifier, a wavelength conversion unit, and alight intensity control unit. The seed laser may be configured to outputseed light that is continuous wave laser light. The light intensitychangeable unit may be configured to change a light intensity of theseed light to thereby make the seed light pulsed and output the pulsedseed light as seed pulse light. The CW excitation laser may beconfigured to output continuous wave excitation light. The amplifier maybe configured to amplify the seed pulse light and output the amplifiedseed pulse light as amplified light, based on an amplification gainincreased by light excitation by the continuous wave excitation light.The wavelength conversion unit may be configured to convert a wavelengthof the amplified light and output harmonic light. The light intensitycontrol unit may be configured to control the light intensity changeableunit according to an input of an external trigger signal. The lightintensity control unit may allow the light intensity changeable unit tooutput the seed pulse light after a certain time elapsed from an inputof the external trigger signal each time the external trigger signal isinput, and allow the light intensity changeable unit to outputsuppression light that suppresses an increase of the amplification gainin a period after an output of the seed pulse light until an input of anext external trigger signal.

A solid state laser system, according to one aspect of the presentdisclosure, may include a first solid-state laser device, a secondsolid-state laser device, and a sum frequency wavelength conversionunit. The first solid-state laser device may include a first seed laser,a first light intensity changeable unit, a first CW excitation laser, afirst amplifier, and a first light intensity control unit. The firstseed laser may be configured to output first seed light that iscontinuous wave laser light. The first light intensity changeable unitmay be configured to change a light intensity of the first seed light tothereby make the first seed light pulsed and output the pulsed firstseed light as first seed pulse light. The first CW excitation laser maybe configured to output first continuous wave excitation light. Thefirst amplifier may be configured to amplify the first seed pulse lightand output the amplified first seed pulse light as first amplifiedlight, based on an amplification gain increased by light excitation bythe first continuous wave excitation light. The first light intensitycontrol unit may be configured to control the first light intensitychangeable unit according to an input of an external trigger signal. Thefirst light intensity control unit may allow the first light intensitychangeable unit to output the first seed pulse light after a certaintime elapsed from an input of the external trigger signal each time theexternal trigger signal is input, and allow the first light intensitychangeable unit to output first suppression light that suppresses anincrease of the amplification gain of the first amplifier in a periodafter an output of the first seed pulse light until an input of a nextexternal trigger signal. The second solid-state laser device may includea second seed laser, a second light intensity changeable unit, a secondCW excitation laser, a second amplifier, and a second light intensitycontrol unit. The second seed laser may be configured to output secondseed light that is continuous wave laser light. The second lightintensity changeable unit may be configured to change a light intensityof the second seed light to thereby make the second seed light pulsedand output the pulsed second seed light as second seed pulse light. Thesecond CW excitation laser may be configured to output second continuouswave excitation light. The second amplifier may be configured to amplifythe second seed pulse light and output the amplified second seed pulselight as second amplified light, based on an amplification gainincreased by light excitation by the second continuous wave excitationlight. The second light intensity control unit may be configured tocontrol the second light intensity changeable unit according to an inputof an external trigger signal. The second light intensity control unitmay allow the second light intensity changeable unit to output thesecond seed pulse light after a certain time elapsed from an input ofthe external trigger signal each time the external trigger signal isinput, and allow the second light intensity changeable unit to outputsecond suppression light that suppresses an increase of theamplification gain of the second amplifier in a period after an outputof the second seed pulse light until an input of a next external triggersignal. The sum frequency wavelength conversion unit may be configuredto generate third pulse laser light including a sum frequency of thefirst pulse laser light output from the first solid-state laser deviceand the second pulse laser light output from the second solid-statelaser device.

A laser device for an exposure device, according to one aspect of thepresent disclosure, may include the solid-state laser system, and anamplifier. The amplifier may include an excimer laser device. Theexcimer laser device may be configured to amplify third pulse laserlight output from the solid-state laser system.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the present disclosure will be described below asmere examples with reference to the accompanying drawings.

FIG. 1 schematically illustrates a configuration of a solid-state laserdevice 10 according to a comparative example;

FIG. 2 is a flowchart illustrating an operation of the solid-state laserdevice 10 illustrated in FIG. 1;

FIGS. 3A to 3H are timing charts illustrating an operation of thesolid-state laser device 10;

FIGS. 4A to 4H are timing charts in the case where a pulse interval T ofexternal trigger signals Tr input to the solid-state laser device 10 is“Tb”;

FIG. 5 schematically illustrates a configuration of a solid-state laserdevice 10 a according to the present disclosure;

FIG. 6 schematically illustrates a configuration of a light intensitycontrol unit 20;

FIG. 7 is a flowchart illustrating an operation of the solid-state laserdevice 10 a illustrated in FIGS. 5 and 6;

FIGS. 8A to 8J are timing charts illustrating an operation of thesolid-state laser device 10 a;

FIG. 9 illustrates a configuration of the light intensity control unit20 according to a first modification;

FIG. 10 is a flowchart illustrating an operation by a control programincorporated in the light intensity control unit 20 according to asecond modification;

FIGS. 11A to 11H are timing charts illustrating an operation of thesolid-state laser device 10 a including the light intensity control unit20 according to a third modification;

FIG. 12 illustrates a configuration of the light intensity control unit20 according to a fourth modification;

FIGS. 13A to 13J are timing charts illustrating an operation of thesolid-state laser device 10 a including the light intensity control unit20 according to the fourth modification;

FIGS. 14A to 14H are timing charts illustrating an operation of thesolid-state laser device 10 a including the light intensity control unit20 according to a sixth modification;

FIGS. 15A to 15H are timing charts illustrating an operation of thesolid-state laser device 10 a in the case where a pulse interval ofexternal trigger signals is acyclic;

FIG. 16 illustrates an exemplary configuration of an optical shutter100;

FIG. 17 illustrates an exemplary configuration of a semiconductoroptical amplifier 200;

FIG. 18 schematically illustrates a configuration of an MOPA-type(Master Oscillator Power Amplifier type) laser device 50 for an exposuredevice;

FIG. 19 schematically illustrates a configuration of a solid-state lasersystem 51 illustrated in FIG. 18;

FIG. 20 schematically illustrates a configuration of an amplifier 54illustrated in FIG. 18;

FIG. 21 schematically illustrates a configuration of an amplifier 300according to a first modification; and

FIG. 22 schematically illustrates a configuration of an amplifier 400according to a second modification.

EMBODIMENTS

Contents

1. Comparative example

-   -   1.1 Configuration    -   1.2 Operation    -   1.3 Problem

2. First Embodiment

-   -   2.1 Configuration    -   2.2 Operation    -   2.3 Effect    -   2.4 First and second signal setting conditions    -   2.5 Definition of wavelength conversion threshold        3. Modifications of light intensity control unit    -   3.1 First modification    -   3.2 Second modification    -   3.3 Third modification    -   3.4 Fourth modification    -   3.5 Fifth modification    -   3.6 Sixth modification        4. Case where pulse interval of external trigger signals is        acyclic        5. Exemplary configuration of light intensity changeable unit    -   5.1 First exemplary configuration        -   5.1.1 Configuration        -   5.1.2 Operation    -   5.2 Second exemplary configuration        -   5.2.1 Configuration        -   5.2.2 Operation            6. Exemplary application of solid-state laser device to a            laser device including MO and amplifier    -   6.1 Configuration    -   6.2 Operation    -   6.3 Effect    -   6.4 Definition of wavelength conversion threshold    -   6.5 Modifications related to wavelength conversion unit    -   6.6 Modifications of amplifier        -   6.6.1 First modification        -   6.6.2 Second modification            7. Other modifications

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the drawings. The embodiments described belowillustrate some examples of the present disclosure, and do not limit thecontents of the present disclosure. All of the configurations and theoperations described in the embodiments are not always indispensable asconfigurations and operations of the present disclosure. It should benoted that the same constituent elements are denoted by the samereference signs, and overlapping description is omitted.

1. Comparative Example

1.1 Configuration

FIG. 1 schematically illustrates a configuration of a solid-state laserdevice 10 according to a comparative example. In FIG. 1, the solid-statelaser device 10 includes a solid-state laser control unit 11, a CWexcitation laser 12, an amplifier 13, a seed laser 14, a light intensitychangeable unit 15, a dichroic mirror 16, a one-shot circuit 17, and awavelength conversion unit 18.

The solid-state laser device 10 is connected with a laser lightirradiation device 2 as an external device. A pulse-state oscillationpreparation signal Rd and a pulse-state external trigger signal Tr areinput to the solid-state laser control unit 11 from the laser lightirradiation control unit 3 included in the laser light irradiationdevice 2.

The CW excitation laser 12 is a light source that outputs continuouswave (CW) excitation light L_(E) that is light having a wavelengthcapable of being optically excited by the amplifier 13. The CWexcitation laser 12 is a semiconductor laser that outputs CW excitationlight L_(E) having a wavelength of about 976 nm, for example. Theamplifier 13 is a solid-state amplifier including an optical crystaldoped with Yb, a fiber amplifier including synthetic quartz doped withYb, or a combination thereof. The CW excitation light L_(E) output fromthe CW excitation laser 12 is reflected at a high rate by the dichroicmirror 16 and is input to the amplifier 13.

The seed laser 14 is a light source that outputs seed light L_(S). Theseed laser 14 is a distributed-feedback semiconductor laser that outputsCW laser light having a wavelength of about 1,060 nm as the seed lightL_(S). It is preferable that the wavelength of the seed light L_(S) isin a wavelength range from 1,020 nm to 1,090 nm.

The light intensity changeable unit 15 is an optical shutter formed of acombination of an electro optical (EO) Pockels cell and a polarizer. Thelight intensity changeable unit 15 is disposed between the seed laser 14and the dichroic mirror 16. The light intensity changeable unit 15 maybe a semiconductor amplifier.

When the one-shot circuit 17 receives an input of the external triggersignal Tr from the solid-state laser control unit 11, the one-shotcircuit 17 generates a pulse signal S having a predetermined pulse widthand outputs it to the light intensity changeable unit 15. When the pulsesignal S is input from the one-shot circuit 17 in a state where the seedlight L_(S) is input from the seed laser 14, the light intensitychangeable unit 15 detects a rising edge of the pulse signal S, andtransmits the seed light L_(S) only during a period corresponding to thepulse width of the pulse signal S. This means that the light intensitychangeable unit 15 outputs seed pulse light L_(P) generated by the seedlight L_(S) made into a pulse, according to the pulse signal S inputfrom the one-shot circuit 17.

The dichroic mirror 16 is disposed at a position where the optical pathof the seed pulse light L_(P) and the optical path of the CW excitationlight L_(E) cross each other, in such a manner that the optical path ofthe seed pulse light L_(P) and the optical path of the CW excitationlight L_(E) substantially match. A surface of the dichroic mirror 16 iscoated with a film at which the CW excitation light L_(E) output fromthe CW excitation laser 12 is reflected at a high rate and which highlytransmits the seed pulse light L_(P) output from the light intensitychangeable unit 15.

The amplifier 13 is optically excited when the CW excitation light L_(E)is input thereto, and in a state where inverted population is formed,the seed pulse light L_(P) is input thereto. When the seed pulse lightL_(P) is input to the amplifier 13, light amplification is caused bystimulated emission. The seed pulse light L_(P) is amplified by theamplifier 13, and is output as amplified light L_(A). When the light isamplified by the stimulated emission, inverted population of theamplifier 13 is consumed.

The wavelength conversion unit 18 includes a non-linear crystal thatconverts the amplified light L_(A) to harmonic light L_(H). The harmoniclight L_(H) is third harmonic light having a wavelength of about 355 nm,or a fourth harmonic light having a wavelength of about 266 nm. When thewavelength conversion unit 18 includes an LBO (LiB₃O₅) crystal, a BBO(β-BaB₂O₄) crystal or the like as a non-linear crystal, third harmoniclight is generated as the harmonic light L_(H). When the wavelengthconversion unit 18 includes an LBO crystal and a CLBO (CsLiB₆O₁₀)crystal as a non-linear crystal, fourth harmonic light is generated asthe harmonic light L_(H).

1.2 Operation

FIG. 2 is a flowchart illustrating the operation of the solid-statelaser device 10 illustrated in FIG. 1. FIGS. 3A to 3H are timing chartsillustrating an operation of the solid-state laser device 10. In thetiming charts of FIGS. 3A and 3D, the vertical axis represents thevoltage value. In the timing charts of FIGS. 3B, 3C, 3E, 3G, and 3H, thevertical axis represents the light intensity. The vertical axis in thetiming chart of FIG. 3F represents an amplification gain.

The solid-state laser control unit 11 included in the solid-state laserdevice 10 controls the operation of the solid-state laser device 10through the processes described below.

First, at S100, the solid-state laser control unit 11 determines whetheror not it has received an oscillation preparation signal Rd from thelaser light irradiation device 2. When the solid-state laser controlunit 11 has not received the oscillation preparation signal Rd (S100;NO), the solid-state laser control unit 11 waits until it receives theoscillation preparation signal Rd. Upon receipt of the oscillationpreparation signal Rd (S100; YES), the solid-state laser control unit 11proceeds to S110.

At S110, the solid-state laser control unit 11 controls the seed laser14 to start laser oscillation operation, and allows the seed laser 14 tooutput the seed light L_(S) having a constant light intensity, asillustrated in FIG. 3B. The seed light L_(S) output from the seed laser14 is input to the light intensity changeable unit 15, and until theexternal trigger signal Tr, described later, is input to the solid-statelaser control unit 11, transmission is suppressed by the light intensitychangeable unit 15.

Next, at S120, the solid-state laser control unit 11 controls the CWexcitation laser 12 to start laser oscillation operation, and allows theCW excitation laser 12 to output the CW excitation light L_(E) havingconstant a light intensity, as illustrated in FIG. 3C. The CW excitationlight L_(E) output from the CW excitation laser 12 is reflected at ahigh rate by the dichroic mirror 16 to thereby be input to the amplifier13, and optically excites the amplifier 13. In the amplifier 13, lightexcitation is generated with the CW excitation light L_(E), wherebyinverted population is formed. The number of atoms in the excited stateof the inverted population is increased as the incident time of the CWexcitation light L_(E) elapses. This means that as illustrated in FIG.3F, the amplification gain of the amplifier 13 is increased as theincident time of the CW excitation light L_(E) elapses.

Next, at S130, the solid-state laser control unit 11 connects theexternal trigger signal line, illustrated by a broken line in FIG. 1, tothe one-shot circuit 17 to thereby enable the external trigger signal Trto be input to the one-shot circuit 17. Accordingly, as illustrated inFIG. 3A, when the external trigger signal Tr is input from the laserlight irradiation device 2 to the solid-state laser control unit 11, theexternal trigger signal Tr is input to the one-shot circuit 17 via theexternal trigger signal line.

At S140, when the external trigger signal Tr is input to one-shotcircuit 17, the one-shot circuit 17 generates a pulse signal S having agiven pulse width as illustrated in FIG. 3D and outputs it to the lightintensity changeable unit 15. When the pulse signal S is input from theone-shot circuit 17, the light intensity changeable unit 15 transmitsthe seed light L_(S) only during the period corresponding to the pulsewidth of the pulse signal S. Thereby, as illustrated in FIG. 3E, seedpulse light L_(P) having a pulse width corresponding to the pulse signalS is output from the light intensity changeable unit 15. The seed pulselight L_(P) passes through, at a high rate, the dichroic mirror 16 andis input to the amplifier 13.

When the seed pulse light L_(P) is input to the amplifier 13, lightamplification is caused by stimulated emission, and the amplified lightL_(A) is output as illustrated in FIG. 3G. The light intensity of theamplified light L_(A) depends on the magnitude of the amplification gainat the point of time when the seed pulse light L_(P) is input to theamplifier 13. The amplified light L_(A) output from the amplifier 13 isinput to the wavelength conversion unit 18. When the amplified lightL_(A) is input to the wavelength conversion unit 18, the amplified lightL_(A) is converted to the harmonic light L_(H) and is output, asillustrated in FIG. 3H. The harmonic light L_(H) is made incident on anillumination optical system, not illustrated, of the laser lightirradiation device 2, as ultraviolet pulse laser light.

The inverted population of the amplifier 13 is consumed when theamplified light L_(A) is generated, and along with it, the amplificationgain is decreased. The amplification gain of the amplifier 13 isincreased again with the incident CW excitation light L_(E), from thepoint of time when the inverted population is consumed by the amplifiedlight L_(A).

At S150, the solid-state laser control unit 11 determines whether or notto stop operation of the solid-state laser device 10. For example, whenthe solid-state laser control unit 11 receives a stop signal, notillustrated, from the laser light irradiation device 2, the solid-statelaser control unit 11 determines to stop operation of the solid-statelaser device 10. A stop signal is input to the solid-state laser device10 when a failure occurs in the solid-state laser device 10 or at thetime of stopping laser oscillation of the solid-state laser device 10.

When determining not to stop operation of the solid-state laser device10 (S150; NO), the solid-state laser control unit 11 keeps the externaltrigger signal line connected with the one-shot circuit 17 to therebyallow the external trigger signal Tr to be input to the one-shot circuit17. The operation of generating the pulse signal S by the one-shotcircuit 17 at S140 and the laser oscillation operation accompanyingthereto are performed each time the external trigger signal Tr is inputto the solid-state laser control unit 11.

When determining to stop the operation of the solid-state laser device10 (S150; YES), the solid-state laser control unit 11 proceeds to S160.At S160, the solid-state laser control unit 11 disconnects the externaltrigger signal line from the one-shot circuit 17. Next, at S170, thesolid-state laser control unit 11 stops the laser oscillation operationof the CW excitation laser 12. Then, at S180, the solid-state lasercontrol unit 11 stops the laser oscillation operation of the seed laser14.

1.3 Problem

FIGS. 3A to 3H are timing charts in the case where the pulse interval Tof the external trigger signals Tr, input to the solid-state laserdevice 10, is “Ta”. Meanwhile, FIGS. 4A to 4H are timing charts in thecase where the pulse interval T of the external trigger signals Tr,input to the solid-state laser device 10, is “Tb”. Here, Tb>Ta. In thesolid-state laser device 10 according to the comparative example, asillustrated in FIGS. 3F and 4F, the amplification gain of the amplifier13 is gradually increased with the CW excitation light L_(E) madeincident thereon, from the point of time when the external triggersignal Tr is input to the solid-state laser device 10 and the invertedpopulation is consumed by the generation of the amplified light L_(A).Accordingly, the magnitude of the amplification gain, at the point oftime when the next external trigger signal Tr is input to thesolid-state laser device 10 and the amplified light L_(A) is generated,depends on the pulse interval T of the external trigger signals Tr.

Accordingly, as the pulse interval T of the external trigger signals Tris larger, that is, as the repetition frequency is lower, the lightintensity of the amplified light L_(A) is higher. The light intensity ofthe amplified light L_(A) in the case of T=Tb as illustrated in FIG. 4Gbecomes higher than that in the case of T=Ta as illustrated in FIG. 3G.

Moreover, as the light intensity of the amplified light L_(A) is higher,the light intensity and the pulse energy of the pulse laser light madeof the harmonic light L_(H) in which the wavelength is converted by thewavelength conversion unit 18 is higher. The light intensity and thepulse energy of the pulse laser light in the case of T=Tb as illustratedin FIG. 4H become higher than those in the case of T=Ta as illustratedin FIG. 3H.

As described above, in the solid-state laser device 10 according to thecomparative example, there is a problem that the light intensity and thepulse energy of the pulse laser light output to the laser lightirradiation device 2 vary depending on the pulse interval T of theexternal trigger signals Tr.

Further, the solid-state laser device 10 according to the comparativeexample is used as a laser device for an exposure device. In that case,the solid-state laser device 10 performs a burst operation in which aburst period and a pause period are alternately repeated according tothe external trigger signal Tr input from the laser light irradiationdevice 2 as an exposure device. The burst period is a period duringwhich pulse laser light is repeatedly output according to the externaltrigger signal Tr, and the pause period is a period during which outputof the pulse laser light is stopped. The burst period is a period duringwhich exposure is performed in one exposure area on a semiconductorwafer in the exposure device. The pause period is a period from the timewhen exposure of one exposure area ends until the time when exposure ofanother exposure area is started. The wafer stage, not illustrated, ismoved in the pause period.

When the solid-state laser device 10 performs the burst operation, thepulse interval T of the external trigger signals Tr becomes longer inthe pause period. Accordingly, the light intensity and the pulse energyof the pulse laser light at the head of the burst, output to the laserlight irradiation device 2 immediately after the start of the burstperiod, are increased. When the light intensity and the pulse energy ofthe pulse laser light are increased, there is a risk that the non-linearcrystal included in the wavelength conversion unit 18 is damaged.Further, in the case where the amplifier 13 is an optical fiberamplifier, there is a risk that the optical fiber is damaged.

2. First Embodiment

Next, a solid-state laser device according to the first embodiment ofthe present disclosure will be described. In the following description,parts that are almost similar to the constituent elements of thesolid-state laser device of the comparative example illustrated in FIG.1 are denoted by the same reference signs and the description thereofmay be omitted as appropriate.

2.1 Configuration

FIG. 5 schematically illustrates a configuration of a solid-state laserdevice 10 a according to the present disclosure. In FIG. 5, thesolid-state laser device 10 a according to the first embodiment includesa light intensity control unit 20 instead of the one-shot circuit 17provided in the configuration of the solid-state laser device 10 of thecomparative example as illustrated in FIG. 1. Further, in the firstembodiment, the solid-state laser control unit 11 is configured suchthat a pulse-state set signal St is input to the light intensity controlunit 20, in addition to the external trigger signal Tr. The otherconstituent elements of the solid-state laser device 10 a are the sameas the constituent elements of the solid-state laser device 10 of thecomparative example.

FIG. 6 schematically illustrates a configuration of the light intensitycontrol unit 20. In FIG. 6, the light intensity control unit 20 includesa delay circuit 21, a first one-shot circuit 22, a second one-shotcircuit 23, an OR circuit 24, a flip-flop (FF) circuit 25, a firstamplifier circuit 26, a second amplifier circuit 27, and an addercircuit 28.

The light intensity control unit 20 outputs a control signal to thelight intensity changeable unit 15 to control the light intensitychangeable unit 15. The light intensity changeable unit 15 changes thelight intensity of the seed light L_(S) input from the seed laser 14 andoutputs it, based on the control signal input from the light intensitycontrol unit 20.

In the solid-state laser device 10 of the comparative example, theone-shot circuit 17 is configured to output the pulse signal S to thelight intensity changeable unit 15, according to the input of theexternal trigger signal Tr. Meanwhile, in the solid-state laser device10 a of the first embodiment, the light intensity control unit 20outputs a first signal S1 and a second signal S2 to the light intensitychangeable unit 15 as control signals, according to the input of theexternal trigger signal Tr. Specifically, each time the external triggersignal Tr is input, the light intensity control unit 20 outputs thefirst signal S1 to the light intensity changeable unit 15 after acertain time elapsed following the input of the external trigger signalTr, and outputs the second signal S2 to the light intensity changeableunit 15 within a period from the output of the first signal S1 until theinput of the next external trigger signal Tr.

The first signal S1 is a pulse signal to be used for generating the seedpulse light L_(P) for allowing the amplifier 13 to generate theamplified light L_(A), similar to the comparative example. The secondsignal S2 is used for generating suppression light L_(D) for suppressingformation of inverted population after the amplified light L_(A) isgenerated.

2.2 Operation

FIG. 7 is a flowchart illustrating an operation of the solid-state laserdevice 10 a illustrated in FIGS. 5 and 6. FIGS. 8A to 8J are timingcharts illustrating an operation of the solid-state laser device 10 a.The solid-state laser control unit 11 included in the solid-state laserdevice 10 a according to the first embodiment controls the operation ofthe solid-state laser device 10 a through the processes described below.

First, at S200, the solid-state laser control unit 11 determines whetheror not it has received the oscillation preparation signal Rd from thelaser light irradiation device 2. When the solid-state laser controlunit 11 has not received the oscillation preparation signal Rd (S200;NO), the solid-state laser control unit 11 waits until it receives theoscillation preparation signal Rd. Upon receipt of the oscillationpreparation signal Rd (S200; YES), the solid-state laser control unit 11proceeds to S210.

At S210, the solid-state laser control unit 11 controls the seed laser14 to start a laser oscillation operation, and allows the seed laser 14to output the seed light L_(S) having a constant light intensity, asillustrated in FIG. 8B. The seed light L_(S) output from the seed laser14 is input to the light intensity changeable unit 15.

Next, at S220, the solid-state laser control unit 11 controls the CWexcitation laser 12 to start laser oscillation operation, and allows theCW excitation laser 12 to output the CW excitation light L_(E) having aconstant light intensity, as illustrated in FIG. 8C. The CW excitationlight L_(E) output from the CW excitation laser 12 is reflected at ahigh rate by the dichroic mirror 16 to thereby be input to the amplifier13, and optically excites the amplifier 13.

Next, at S230, the solid-state laser control unit 11 transmits the setsignal St to the light intensity control unit 20. When the set signal Stis input, the light intensity control unit 20 outputs the second signalS2 as illustrated in FIG. 8D. Specifically, as illustrated in FIG. 6,when the set signal St is input to the light intensity control unit 20,the set signal St is input to a set terminal S of the FF circuit 25 viathe OR circuit 24. At that time, as a signal is not input to a resetterminal R of the FF circuit 25, the FF circuit 25 detects a rising edgeof the set signal St input to the set terminal S, and becomes in a setstate, as illustrated in FIG. 8I. When the FF circuit 25 is in a setstate, an output signal value of an output terminal Q takes “1”. Theoutput signal of the output terminal Q is input to the first amplifiercircuit 26.

The first amplifier circuit 26 amplifies a signal input from the outputterminal Q of the FF circuit 25. When the signal value input from theoutput terminal Q of the FF circuit 25 is “1”, the first amplifiercircuit 26 amplifies it and outputs the second signal S2. An outputsignal of the first amplifier circuit 26 is input to a first inputterminal In1 of the adder circuit 28, and an output signal of the secondamplifier circuit 27 is input to a second input terminal In2. The addercircuit 28 adds the output signals of the first and second amplifiercircuits 26 and 27 that are analog signals, and outputs the resultantfrom an output terminal Out.

As illustrated in FIG. 8J, when the second signal S2 is input from thefirst amplifier circuit 26 to the first input terminal In1 of the addercircuit 28, a signal value input to the second input terminal In2 fromthe second amplifier circuit 27 is “0”. In that case, the second signalS2 is output from the output terminal Out of the adder circuit 28. Thus,the second signal S2 is output from the light intensity control unit 20and is input to the light intensity changeable unit 15.

As the signal value input to the light intensity changeable unit 15 islarger, the light transmittance with respect to the seed light L_(S)input from the seed laser 14 becomes larger. The second signal S2 has asmaller signal value than that of the first signal S1 described below.As such, when the second signal S2 is input, the light intensitychangeable unit 15 has lower light transmittance than that at the timewhen the first signal S1 is input, and transmits part of the seed lightL_(S). Accordingly, when the second signal S2 is input to the lightintensity control unit 20, the suppression light L_(D) in which thelight intensity of the seed light L_(S) is lowered is output from thelight intensity control unit 20 as illustrated in FIG. 8E. In thepresent embodiment, the suppression light L_(D) is CW laser light havinga constant light intensity.

The suppression light L_(D) output from the light intensity control unit20 passes through, at a high rate, the dichroic mirror 16 and is inputto the amplifier 13. At that time, in the amplifier 13, while lightexcitation is caused by receiving the CW excitation light L_(E), opticalinteraction is caused by an input of the suppression light L_(D) wherebystimulated emission of light is caused. As such, formation of invertedpopulation is suppressed as illustrated in FIG. 8F. Thereby, an increasein the amplification gain is suppressed. As illustrated in FIG. 8G,output light L_(B) output from the amplifier 13 by the input of thesuppression light L_(D) has a low light intensity. The output lightL_(B) is output light secondarily generated by the suppression lightL_(D) in the amplifier 13. Accordingly, it is referred to as secondarylight L_(B) hereinafter.

The secondary light L_(B) is input to the wavelength conversion unit 18.The wavelength conversion unit 18 has a wavelength conversion efficiencyof a given value or higher with respect to incident light having a lightintensity of a wavelength conversion threshold I_(C) or larger, and hasa wavelength conversion efficiency less than the given value withrespect to incident light having a light intensity smaller than thewavelength conversion threshold I_(C). As illustrated in FIG. 8G, thelight intensity of the secondary light L_(B) input to the wavelengthconversion unit 18 is less than the wavelength conversion thresholdI_(C), and the wavelength conversion is suppressed. Accordingly, asillustrated in FIG. 8H, in the period during which the second signal S2is output from the light intensity control unit 20, output of harmoniclight from the wavelength conversion unit 18 is suppressed. In the firstamplifier circuit 26, the amplification rate is set so as to generatethe second signal S2 in which the light intensity of the secondary lightL_(B) generated in the amplifier 13 becomes less than the wavelengthconversion threshold I_(C).

Next, at S240, the solid-state laser control unit 11 connects theexternal trigger signal line, illustrated by a broken line in FIG. 5, tothe light intensity control unit 20 to thereby enable the externaltrigger signal Tr to be input to the light intensity control unit 20.Accordingly, as illustrated in FIG. 8A, when the external trigger signalTr is input from the laser light irradiation device 2 to the solid-statelaser control unit 11, the external trigger signal Tr is input to thelight intensity control unit 20 via the external trigger signal line.

At S250, when the external trigger signal Tr is input, the lightintensity control unit 20 generates the first signal S1 after a certaintime Td elapses, and outputs it to the light intensity changeable unit15, as illustrated in FIG. 8D. Specifically, as illustrated in FIG. 6,when the external trigger signal Tr is input to the light intensitycontrol unit 20, the external trigger signal Tr is input to the delaycircuit 21 and also input to the reset terminal R of the FF circuit 25.

As illustrated in FIG. 8I, the FF circuit 25 detects a rising edge ofthe external trigger signal Tr input to the reset terminal R, andbecomes in a reset state. When the FF circuit 25 is in a reset state, anoutput signal value of the output terminal Q takes “0”. The outputsignal of the output terminal Q is input to the first input terminal In1of the adder circuit 28 via the first amplifier circuit 26. At thattime, a signal value input to the second input terminal In2 is “0”.Accordingly, as illustrated in FIG. 8J, the signal value output from theoutput terminal Out of the adder circuit 28 is “0”, which means a groundsignal. The ground signal is output from the light intensity controlunit 20 and is input to the light intensity changeable unit 15.

When the ground signal is input to the light intensity changeable unit15, the light transmittance becomes almost “0”, whereby transmission ofthe seed light L_(S) input from the seed laser 14 is suppressed.Thereby, as illustrated in FIG. 8D, light is not input from the lightintensity changeable unit 15 to the amplifier 13. Accordingly, theamplifier 13 receives the CW excitation light L_(E) whereby theamplification gain is gradually increased, as illustrated in FIG. 8F.

On the other hand, the external trigger signal Tr input to the delaycircuit 21 is delayed by the certain time Td by the delay circuit 21,and is output. The external trigger signal Tr output from the delaycircuit 21 is input to the first one-shot circuit 22. The first one-shotcircuit 22 detects a rising edge of the input external trigger signalTr. When detecting the rising edge of the external trigger signal Tr,the first one-shot circuit 22 generates a pulse signal having a givenpulse width Δt, and inputs it to the second amplifier circuit 27 and thesecond one-shot circuit 23.

The second amplifier circuit 27 amplifies the input pulse signal togenerate the first signal S1 having a larger voltage value than that ofthe second signal S2, and outputs it. The first signal S1 has a pulsewidth Δt. The pulse width Δt is preferably in a range from 0.001 ns to100 ns.

As illustrated in FIG. 8J, the first signal S1 is input to the secondinput terminal In2 of the adder circuit 28. At that time, a signal valueinput to the input terminal In1 is “0”. In that case, the first signalS1 is output from the output terminal Out of the adder circuit 28. Thus,the first signal S1 is output from the light intensity control unit 20and is input to the light intensity changeable unit 15. This means thatthe first signal S1 is input to the light intensity changeable unit 15after the certain time Td elapsed from the time when the externaltrigger signal Tr is input to the light intensity control unit 20.

When the first signal S1 is input, the light intensity changeable unit15 transmits the seed light L_(S) input from the seed laser 14. Thereby,as illustrated in FIG. 8E, the seed pulse light L_(P) having a pulsewidth corresponding to the first signal S1 is output from the lightintensity changeable unit 15. The seed pulse light L_(P) passes through,at a high rate, the dichroic mirror 16 and is input to the amplifier 13.

When the seed pulse light L_(P) is input to the amplifier 13, lightamplification is caused by stimulated emission, and the amplified lightL_(A) is output as illustrated in FIG. 8G. The light intensity of theamplified light L_(A) depends on the magnitude of the amplification gainafter the certain time Td elapsed from the time when the externaltrigger signal Tr is input to the light intensity control unit 20. Theamplified light L_(A) output from the amplifier 13 has a light intensityof the wavelength conversion threshold I_(C) or larger, and is input tothe wavelength conversion unit 18. When the amplified light L_(A) isinput to the wavelength conversion unit 18, the amplified light L_(A) isconverted to the harmonic light L_(H) and is output, as illustrated inFIG. 8H. The harmonic light L_(H) is made incident on an illuminationoptical system, not illustrated, of the laser light irradiation device2, as ultraviolet pulse laser light.

Meanwhile, the second one-shot circuit 23 detects a falling edge of thepulse signal output from the first one-shot circuit 22. When detectingthe falling edge of the pulse signal, the second one-shot circuit 23generates the pulse-state set signal St, and outputs it. The set signalSt is input to the set terminal S of the FF circuit 25 via the ORcircuit 24. At that time, as a signal is not input to the reset terminalR of the FF circuit 25, the FF circuit 25 detects a rising edge of theset signal St input to the set terminal S, and becomes in a set state,as illustrated in FIG. 8I.

As a result, the second signal S2 is output from the output terminal Outof the adder circuit 28, similar to the case of S230. Then, when thesecond signal S2 is input to the light intensity control unit 20, thesuppression light L_(D) is output from the light intensity control unit20 as illustrated in FIG. 8E. With the suppression light L_(D), anincrease of the amplification gain of the amplifier 13 is suppressed,and the secondary light L_(B) is output from the amplifier 13. As thesecondary light L_(B) has a light intensity smaller than the wavelengthconversion threshold I_(C), wavelength conversion in the wavelengthconversion unit 18 is suppressed, and output of the harmonic light issuppressed.

At S260, the solid-state laser control unit 11 determines whether or notto stop operation of the solid-state laser device 10 a. For example,when the solid-state laser control unit 11 receives a stop signal, notillustrated, from the laser light irradiation device 2, the solid-statelaser control unit 11 determines to stop operation of the solid-statelaser device 10 a. A stop signal is input to the solid-state lasercontrol unit 11 when a failure occurs in the solid-state laser device 10a or at the time of stopping laser oscillation of the solid-state laserdevice 10 a.

When determining not to stop operation of the solid-state laser device10 a (S260; NO), the solid-state laser control unit 11 keeps theexternal trigger signal line connected with the light intensity controlunit 20 to thereby allow the external trigger signal Tr to be input tothe light intensity control unit 20. The operation of generating thefirst and second signals S1 and S2 by the light intensity control unit20 at S250 and the laser oscillation operation accompanying thereto areperformed each time the external trigger signal Tr is input to thesolid-state laser control unit 11.

When determining to stop operation of the solid-state laser device 10 a(S260; YES), the solid-state laser control unit 11 proceeds to S270. AtS270, the solid-state laser control unit 11 disconnects the externaltrigger signal line from the light intensity control unit 20. Next, atS280, the solid-state laser control unit 11 stops the laser oscillationoperation of the CW excitation laser 12. Then, at S290, the solid-statelaser control unit 11 stops the laser oscillation operation of the seedlaser 14.

The certain time Td that is a delay time of a signal by the delaycircuit 21 is set to be shorter than the pulse interval T of theexternal trigger signals Tr. The certain time Td is preferably in arange from 10 μs to 1,000 μs. The certain time Td is a fixed valuerather than a value that varies according to the pulse interval T of theexternal trigger signals Tr. The fixed value may be set to the delaycircuit 21 by the solid-state laser control unit 11 according to thepulse energy of the harmonic light L_(H) as the pulse laser light outputfrom the solid-state laser device 10 a to the laser light irradiationdevice 2. For example, the solid-state laser control unit 11 sets thefixed value to be a larger value as the required pulse energy of theharmonic light L_(H) output from the solid-state laser device 10 a ishigher.

2.3 Effect

According to the solid-state laser device 10 a of the first embodiment,an increase of the amplification gain of the amplifier 13 begins at thepoint of time when the external trigger signal Tr is input to thesolid-state laser device 10 a, and the amplified light L_(A) isgenerated by the amplification gain after the certain time Td elapsed.Accordingly, in the first embodiment, under a condition that therelationship of “T>Td” is satisfied, the light intensity of theamplified light L_(A) is constant regardless of the pulse interval T ofthe external trigger signal Tr. This means that the light intensity andthe pulse energy of the pulse laser light output to the laser lightirradiation device 2 are constant regardless of the pulse interval T.

Further, in the first embodiment, the light intensity and the pulseenergy of the pulse laser light, at the head of the burst output to thelaser light irradiation device 2 immediately after the start of theburst period after the pause period of the burst operation, do notincrease. This prevents the non-linear crystal included in thewavelength conversion unit 18 from being damaged. Further, in the casewhere the amplifier 13 is an optical fiber amplifier, a damage of theoptical fiber is suppressed.

2.4 First and Second Signal Setting Conditions

In the first embodiment, while the suppression light L_(D) is set tohave a light intensity lower than that of the seed pulse light L_(P), itis only necessary that the light intensity of the suppression lightL_(D) may have magnitude by which light excitation generated by theamplifier 13 receiving the CW excitation light L_(E) after generation ofthe amplified light L_(A) is consumed and formation of invertedpopulation can be suppressed.

Specifically, when the light intensity of the seed pulse light L_(P)output from the light intensity changeable unit 15 according to thefirst signal S1 is represented by I_(P), and the light intensity of thesuppression light L_(D) output from the light intensity changeable unit15 according to the second signal S2 is represented by I_(D), it ispreferable that the light intensity ratio I_(D)/I_(P) satisfies thefollowing Expression (1):

0.1<I _(D) /I _(P)<1.0  (1)

This means that when the voltage value of the first signal S1 generatedby the light intensity control unit 20 is represented by V_(S1), and thevoltage value of the second signal S2 is represented by V_(S2), it ispreferable that the voltage ratio V_(S2)/V_(S1) satisfies the followingExpression (2):

0.1<V _(S2) /V _(S1)<1.0  (2)

In the first embodiment, while the light intensity control unit 20includes the first and second amplifier circuits 26 and 27 and the addercircuit 28, when V_(S1)=V_(S2), a configuration described below is alsopossible. For example, with respect to the configuration of the lightintensity control unit 20 illustrated in FIG. 6, an OR circuit is added,the first and second amplifier circuits 26 and 27 are made common, andthe adder circuit 28 is deleted. An output from the output terminal Q ofthe FF circuit 25 and an output from the first one-shot circuit 22 areinput to the OR circuit. An output of the OR circuit is input to thecommon amplifier circuit.

Further, in the first embodiment, the light intensity control unit 20outputs a ground signal to the light intensity changeable unit 15 duringa period after the external trigger signal Tr is input until the certaintime Td elapsed, to thereby make the light transmittance of the lightintensity changeable unit 15 almost “0”. It is only necessary that thelight intensity control unit 20 allows the light intensity changeableunit 15 to suppress transmission of the seed light L_(S) so as toincrease the amplification gain of the amplifier 13. The signal valueinput to the light intensity changeable unit 15 and the lighttransmittance of the light intensity changeable unit 15 may not be “0”.

2.5 Definition of Wavelength Conversion Threshold

In the wavelength conversion unit 18, the wavelength conversionefficiency of the incident light, from the amplifier 13, to the harmoniclight depends on the light intensity of the incident light. Thewavelength conversion efficiency is lower as the light intensity of theincident light is lower. The wavelength conversion threshold I_(C) isdefined as a light intensity of incident light in which the wavelengthconversion efficiency takes a given value. It is preferable that thegiven value is in a range from 1% to 2%, for example. It is alsopreferable that the given value is in a range from 0% to 0.01%.

The wavelength conversion threshold I_(C) may be defined in thewavelength conversion unit 18 based on a ratio of a second lightintensity that is the light intensity of harmonic light generatedaccording to the second signal S2 to a first light intensity that is thelight intensity of the harmonic light L_(H) generated according to thefirst signal S1. For example, the wavelength conversion threshold I_(C)may be defined as a light intensity in which the ratio of the secondlight intensity to the first light intensity is in a range from 0% to10%.

3. Modifications of Light Intensity Control Unit

Various modifications of the light intensity control unit 20 can bemade. Hereinafter, modifications of the light intensity control unit 20will be described.

3.1 First Modification

Next, a first modification of the light intensity control unit 20 willbe described. As illustrated in FIG. 8D, the light intensity controlunit 20 of the first embodiment is configured to generate the secondsignal S2 immediately after generating the first signal S1. As the firstmodification, description will be given on the case where the lightintensity control unit 20 is configured to generate the second signal S2after a predetermined time elapsed from generation of the first signalS1.

FIG. 9 illustrates a configuration of the light intensity control unit20 according to the first modification. In FIG. 9, the light intensitycontrol unit 20 according to the first modification includes a delaycircuit 29, in addition to the constituent elements of the lightintensity control unit 20 of the first embodiment illustrated in FIG. 6.The delay circuit 29 is disposed between the first one-shot circuit 22and the second one-shot circuit 23, and delays the input timing of apulse signal to the second one-shot circuit 23 by a given period oftime. The delay circuit 29 may be disposed between the second one-shotcircuit 23 and the OR circuit 24. The other configurations andoperations of the light intensity control unit 20 according to the firstmodification are similar to those of the light intensity control unit 20of the first embodiment.

3.2 Second Modification

Next, a second modification of the light intensity control unit 20 willbe described. In the first embodiment, the light intensity control unit20 is configured of a logical circuit. Meanwhile, as the secondmodification, description will be given on the case where the lightintensity control unit 20 is configured of an integrated circuit capableof programming a control program such as a field programmable gate array(FPGA).

FIG. 10 is a flowchart illustrating an operation of a control programincorporated in the light intensity control unit 20 according to thesecond modification. The light intensity control unit 20 performs theprocesses described below based on the control program.

First, at S300, the light intensity control unit 20 determines whetheror not the set signal St has been received from the solid-state lasercontrol unit 11. When the light intensity control unit 20 has notreceived the set signal St (S300; NO), the light intensity control unit20 waits until it receives the set signal St. Upon receipt of the setsignal St (S300; YES), the light intensity control unit 20 proceeds toS310.

At S310, the light intensity control unit 20 outputs the second signalS2 to the light intensity changeable unit 15. Then, at S320, the lightintensity control unit 20 determines whether or not it has received theexternal trigger signal Tr from the solid-state laser control unit 11.When the external trigger signal Tr has not been received (S320; NO),the light intensity control unit 20 continues outputting the secondsignal S2 of S310. Upon receipt of the external trigger signal Tr (S320;YES), the light intensity control unit 20 stops outputting the secondsignal S2, and proceeds to S330.

At S330, the light intensity control unit 20 resets a timer T1 andstarts time measuring. Next, at S340, the light intensity control unit20 outputs a ground signal (0V) to the light intensity changeable unit15. Then, at S350, the light intensity control unit 20 determineswhether or not an elapsed time T1 from the start of time measuring atS330 reaches the certain time Td. When the elapsed time T1 does notreach the certain time Td (S350; NO), the light intensity control unit20 continues outputting the ground signal of S340. When the elapsed timeT1 reaches the certain time Td (S350; YES), the light intensity controlunit 20 stops outputting the ground signal, and proceeds to S360.

At S360, the light intensity control unit 20 resets a timer T2 andstarts time measuring. Next, at S370, the light intensity control unit20 outputs the first signal S1 to the light intensity changeable unit15. Then, at S380, the light intensity control unit 20 determineswhether or not an elapsed time T2 from the start of time measuring atS360 reaches a time corresponding to the pulse width Δt. When theelapsed time T2 does not reach the time corresponding to the pulse widthΔt (S380; NO), the light intensity control unit 20 continues outputtingthe first signal S1 of S370. When the elapsed time T2 reaches the timecorresponding to the pulse width Δt (S380; YES), the light intensitycontrol unit 20 stops outputting the first signal S1, and proceeds toS310. After this process, the light intensity control unit 20 repeatsthe processes of S310 to S380.

The other configurations and operations of the light intensity controlunit 20 according to the second modification are similar to those of thelight intensity control unit 20 of the first embodiment.

As described above, the light intensity control unit 20 according to thesecond modification performs the aforementioned processes based on thecontrol program to thereby enable the first and second signals S1 and S2to be output to the light intensity changeable unit 15 at the timingthat is the same as that of the light intensity control unit 20 of thefirst embodiment. In the case of generating the second signal S2 afterthe given time elapsed from generation of the first signal S1 as in thecase of the first modification, it is only necessary to move to S310after the given time elapsed from the point of time when YESdetermination is made at S380.

3.3 Third Modification

Next, a third modification of the light intensity control unit 20 willbe described. In the first embodiment, there is a possibility thatinverted population of the amplifier 13 is not completely consumed bythe seed pulse light L_(P) generated by the light intensity changeableunit 15 based on the first signal S1, and that the amplification gainremains after the generation of the amplified light L_(A). When theamplification gain remaining after the generation of the amplified lightL_(A) is large, there is a possibility that the suppression light L_(D)generated by the light intensity changeable unit 15 based on the secondsignal S2 is amplified, and that the light intensity of the resultantsecondary light L_(B) exceeds the wavelength conversion threshold I_(C).Accordingly, in the first embodiment, wavelength conversion may becaused in the wavelength conversion unit 18 after the harmonic lightL_(H) is output corresponding to the first signal S1, and harmonic lightaccompanying thereto may be output.

FIGS. 11A to 11H are timing charts illustrating an operation of thesolid-state laser device 10 a including the light intensity control unit20 according to the third modification. The light intensity control unit20 according to the third modification is configured to, afteroutputting the first signal S1 to the light intensity changeable unit15, gradually increase the voltage value of the second signal S2 to beoutput to the light intensity changeable unit 15 during a certain periodTc, as illustrated in FIG. 11D. Thereby, in the suppression light L_(D)generated by the light intensity changeable unit 15, the light intensityis gradually increased during the certain period Tc after generation ofthe seed pulse light L_(P) as illustrated in FIG. 11E.

The other configurations and operations of the light intensity controlunit 20 according to the third modification are similar to those of thelight intensity control unit 20 of the first embodiment.

In the solid-state laser device 10 a including the light intensitycontrol unit 20 according to the third modification, when theamplification gain remains after generation of the amplified lightL_(A), the remaining amplification gain is gradually decreased by thesuppression light L_(D) as illustrated in FIG. 11F. Accordingly, asillustrated in FIG. 11G, the suppression light L_(D) is amplified by theamplification gain remaining after the generation of the amplified lightL_(A), whereby the light intensity of the secondary light L_(B) isprevented from exceeding the wavelength conversion threshold I_(C).Consequently, as illustrated in FIG. 11H, it is possible to preventwavelength conversion from being caused in the wavelength conversionunit 18 after the harmonic light L_(H) is output corresponding to thefirst signal S1, and to prevent harmonic light accompanying thereto frombeing output.

3.4 Fourth Modification

Next, a fourth modification of the light intensity control unit 20 willbe described. The light intensity control unit 20 of the firstembodiment outputs the second signal S2 having a constant voltage value.Meanwhile, as the fourth modification, description will be given on thecase where the light intensity control unit 20 is configured to outputthe second signal S2 configured of a continuous pulse.

FIG. 12 illustrates a configuration of the light intensity control unit20 according to the fourth modification. In FIG. 12, the light intensitycontrol unit 20 according to the fourth modification includes an ANDcircuit 30 and a pulse generator 31, in addition to the constituentelements of the light intensity control unit 20 of the first embodimentillustrated in FIG. 6. The AND circuit 30 is disposed between the FFcircuit 25 and the first amplifier circuit 26. A first input terminal ofthe AND circuit 30 is connected with the output terminal Q of the FFcircuit 25 via a signal line. A second input terminal of the AND circuit30 is connected with the pulse generator 31 via a signal line. An outputterminal of the AND circuit 30 is connected with the input terminal ofthe first amplifier circuit 26 via a signal line.

The pulse generator 31 generates a continuous pulse signal Ps having agiven pulse period Ts. The AND circuit 30 outputs a logical product ofan output signal value of the output terminal Q of the FF circuit 25 andthe continuous pulse signal Ps. Accordingly, the AND circuit 30 outputs“0” when the output signal value of the output terminal Q is “0”, whilethe AND circuit 30 outputs the continuous pulse signal Ps when theoutput signal value of the output terminal Q is “1”. The continuouspulse signal Ps is input to the first amplifier circuit 26, and thesecond signal S2 configured of continuous pulses having the pulse periodTs is generated by the first amplifier circuit 26.

FIGS. 13A to 13J are timing charts illustrating an operation of thesolid-state laser device 10 a including the light intensity control unit20 according to the fourth modification. As illustrated in FIG. 13D, thelight intensity control unit 20 of the fourth modification outputs thesecond signal S2 configured of continuous pulses to the light intensitychangeable unit 15, after outputting the first signal S1. As illustratedin FIG. 13E, the suppression light L_(D) output from the light intensitychangeable unit 15 based on the second signal S2 becomes pulse-statepulse laser light. As illustrated in FIG. 13F, with the pulse-statesuppression light L_(D), an increase of the amplification gain of theamplifier 13 is suppressed. As described above, when the increase of theamplification gain is suppressed, the light intensity of the secondarylight L_(B) becomes smaller than the wavelength conversion thresholdI_(C) of the wavelength conversion unit 18 as illustrated in FIG. 13G,whereby the wavelength conversion with respect to the secondary lightL_(B) is suppressed.

As the pulse period Ts of the continuous pulse signal Ps is larger, theamount of increase of the amplification gain illustrated in FIG. 13F islarger. Thereby, the pulse period Ts must be a certain value or lower.It is preferable that the pulse period Ts is 1/10 of the certain time Tdor shorter, and it is more preferable that it is 1/100 of the certaintime Td or shorter.

The other configurations and operations of the light intensity controlunit 20 according to the fourth modification are similar to those of thelight intensity control unit 20 of the first embodiment.

3.5 Fifth Modification

Next, a fifth modification of the light intensity control unit 20 willbe described. In the fourth modification, the light intensity controlunit 20 is configured of a logical circuit. Meanwhile, as the fifthmodification, description will be given on the case where the lightintensity control unit 20 according to the fourth modification isconfigured of an integrated circuit capable of programming a controlprogram such as a field programmable gate array (FPGA).

A flowchart of a control program incorporated in the light intensitycontrol unit 20 of the fifth modification is similar to the flowchartillustrated in FIG. 10 according to the second modification. In thefifth modification, at S310, the light intensity control unit 20 iscontrolled to output the second signal S2 as the continuous pulse signalPs.

The other configurations and operations of the light intensity controlunit 20 according to the fifth modification are similar to those of thelight intensity control unit 20 of the fourth modification.

3.6 Sixth Modification

Next, a sixth modification of the light intensity control unit 20 willbe described. In the fourth and fifth modifications, voltage values ofrespective pulses output as the second signal S2 by the light intensitycontrol unit 20 are constant. Even in the fourth and fifthmodifications, when the amplification gain remaining after thegeneration of the amplified light L_(A) is large, there is a possibilitythat the suppression light L_(D) generated by the light intensitychangeable unit 15 based on the second signal S2 is amplified, and thatthe light intensity of the secondary light L_(B) exceeds the wavelengthconversion threshold I_(C).

FIGS. 14A to 14H are timing charts illustrating an operation of thesolid-state laser device 10 a including the light intensity control unit20 according to the sixth modification. The light intensity control unit20 according to the sixth modification is configured to, afteroutputting the first signal S1 to the light intensity changeable unit15, gradually increase the voltage value of each pulse of the secondsignal S2 to be output to the light intensity changeable unit 15 duringthe certain period Tc, as illustrated in FIG. 14D. Thereby, in thepulse-state suppression light L_(D) generated by the light intensitychangeable unit 15, the light intensity is gradually increased duringthe certain period Tc after generation of the seed pulse light L_(P), asillustrated in FIG. 14E.

The other configurations and operations of the light intensity controlunit 20 according to the sixth modification are similar to those of thelight intensity control unit 20 of the fourth modification.

In the solid-state laser device 10 a including the light intensitycontrol unit 20 according to the sixth modification, when theamplification gain remains after generation of the amplified lightL_(A), the remaining amplification gain is gradually decreased by thesuppression light L_(D) as illustrated in FIG. 14F. Accordingly, asillustrated in FIG. 14G, the suppression light L_(D) is amplified by theamplification gain remaining after the generation of the amplified lightL_(A), whereby the light intensity of the secondary light L_(B) isprevented from exceeding the wavelength conversion threshold I_(C).Consequently, it is possible to prevent wavelength conversion from beingcaused in the wavelength conversion unit 18 after the harmonic lightL_(H) is output corresponding to the first signal S1, and to preventharmonic light accompanying thereto from being output, as illustrated inFIG. 14H.

4. Case where Pulse Interval of External Trigger Signal is Acyclic

The first embodiment and the respective modifications of the presentdisclosure illustrate an example in which, during the burst period, theexternal trigger signals Tr are cyclically input from the laser lightirradiation device 2 to the solid-state laser control unit 11 with aconstant pulse interval T. However, the pulse interval T may be acyclic.

FIGS. 15A to 15H are timing charts in the case where the pulse intervalin the burst period of the external trigger signals Tr output from thelaser light irradiation device 2 to the solid-state laser control unit11 is acyclic, in the solid-state laser device 10 of the firstembodiment. In this drawing, pulse intervals T1 to T4 of the externaltrigger signals Tr have a relation satisfying T1>T2>T3>T4. However, thepresent invention is not limited to this relation. It is only necessarythat the pulse interval of the external trigger signal Tr is larger thanthe certain time Td.

In the solid-state laser device 10 a, even in the case where the pulseinterval of the external trigger signals Tr is acyclic, the period inwhich the amplification gain is increased in the amplifier 13 is thecertain time Td, similar to the case where the pulse interval isconstant. Accordingly, the light intensity of the amplified light L_(A)is constant. This means that the light intensity and the pulse energy ofthe pulse laser light output to the laser light irradiation device 2 areconstant even when the pulse interval is acyclic.

Even in the respective modifications, the light intensity and the pulseenergy of the pulse laser light output to the laser light irradiationdevice 2 are also constant, even when the pulse interval of the externaltrigger signals Tr is acyclic.

5. Exemplary Configuration of Light Intensity Changeable Unit

The light intensity changeable unit 15 can be configured in variousways. Specific exemplary configurations of the light intensitychangeable unit 15 will be described below.

5.1 First Exemplary Configuration

Next, a specific configuration of an optical shutter 100 applicable asthe light intensity changeable unit 15 will be described with referenceto FIG. 16.

5.1.1 Configuration

FIG. 16 illustrates an exemplary configuration of the optical shutter100. The optical shutter 100 includes a Pockels cell 110 and a polarizer120. The Pockels cell 110 includes a high-voltage power supply 111, afirst electrode 112 a, a second electrode 112 b, and an electro opticalcrystal 113. The first electrode 112 a and the second electrode 112 bare disposed opposite to each other, with the electro optical crystal113 being interposed between them. The first electrode 112 a isconnected with the high-voltage power supply 111. The second electrode112 b is grounded.

In the case of applying the optical shutter 100 as the light intensitychangeable unit 15, the high-voltage power supply 111 is controlled bythe light intensity control unit 20. The seed light L_(S) output fromthe seed laser 14 is incident on the electro optical crystal 113. Theoutput light from the optical shutter 100 is made incident on theamplifier 13 via the dichroic mirror 16. The seed light L_(S) madeincident on the electro optical crystal 113 is linearly polarized lightin which the polarization direction is vertical to the sheet surface.

5.1.2 Operation

The high-voltage power supply 111 applies high voltage between the firstelectrode 112 a and the second electrode 112 b, according to a voltagevalue of a control signal input from the light intensity control unit20. When high voltage corresponding to the maximum voltage value of thecontrol signal is applied between the first electrode 112 a and thesecond electrode 112 b, the electro optical crystal 113 exhibits anaction equivalent to a λ/2 plate with respect to the incident light.

When high voltage is not applied to the electro optical crystal 113, theseed light Ls of the linearly polarized light in which the polarizationdirection is vertical to the sheet surface passes through the electrooptical crystal 113 in the polarized state as it is, and is reflected bythe polarizer 120. In that case, the seed light L_(S) is not output fromthe optical shutter 100. This means that, in that case, the lighttransmittance of the optical shutter 100 with respect to the seed lightL_(S) is almost 0%. In FIG. 16, regarding the linearly polarized lightin which the polarization direction is vertical to the sheet surface,the optical path thereof is indicated by solid lines, and thepolarization direction is indicated by black dots.

Meanwhile, when high voltage corresponding to the maximum voltage valueof the control signal is applied to the electro optical crystal 113, thephase of the seed light Ls is shifted by λ/2 when it passes through theelectro optical crystal 113, and the polarization direction is convertedto a direction including the sheet surface. In that case, the seed lightL_(S) passes through the polarizer 120 and is output from the opticalshutter 100. This means that, in that case, the light transmittance ofthe optical shutter 100 with respect to the seed light L_(S) is almost100%. In FIG. 16, regarding the linearly polarized light in which thepolarization direction is a direction including the sheet surface, theoptical path thereof is indicated by a broken line, and the polarizationdirection is indicated by arrows.

Further, by changing the control signal applied between the firstelectrode 112 a and the second electrode 112 b, the light transmittanceof the optical shutter 100 with respect to the seed light L_(S) can bechanged between 0% and 100%.

In the first embodiment and the respective modifications, the firstsignal S1, the second signal S2, or the ground signal (0V) is input asthe control signal from the light intensity control unit 20 to theoptical shutter 100. The voltage value V_(S1) of the first signal S1corresponds to the maximum voltage value. When the voltage value V_(S1)is input to the high-voltage power supply 111, the light transmittanceof the optical shutter 100 becomes almost 100%. Meanwhile, the voltagevalue V_(S2) of the second signal S2 is lower than the maximum voltagevalue. Therefore, when the voltage value V_(S2) is input to thehigh-voltage power supply 111, the light transmittance of the opticalshutter 100 takes a value that is less than 100% and corresponds to thevoltage value V_(S2). Further, when the ground signal is input to thehigh-voltage power supply 111, the light transmittance of the opticalshutter 100 becomes almost 0%.

This means that the seed pulse light L_(P) is generated when the firstsignal S1 is input from the light intensity control unit 20 to theoptical shutter 100, and the suppression light L_(D) is generated whenthe second signal S2 is input.

The Pockels cell 110 has responsiveness of about 1 ns, and is able tochange the light transmittance at high speed. As the optical shutter100, an acousto-optic element may be used. As the acousto-optic elementhas responsiveness of about several 100 ns, it is applicable as thelight intensity changeable unit 15.

The optical shutter 100 of FIG. 16A may further include a polarizer anda λ/2 plate on the optical path on the upstream side, and function as anoptical isolator. In FIG. 16, the left side is the upstream side and theright side is the downstream side. In that case, when given high voltageis applied between the first electrode 112 a and the second electrode112 b of the Pockels cell 110, the optical isolator transmits light at ahigh rate from both the upstream side and the downstream side. Thismeans that the optical isolator becomes in an open state. On thecontrary, when given high voltage is not applied between the firstelectrode 112 a and the second electrode 112 b, transmission of lightfrom both the upstream side and the downstream side is suppressed. Thismeans that the optical isolator is in a closed state.

5.2 Second Exemplary Configuration

Next, a specific configuration of a semiconductor optical amplifier 200applicable as the light intensity changeable unit 15 will be describedwith reference to FIG. 17.

5.2.1 Configuration

FIG. 17 illustrates an exemplary configuration of the semiconductoroptical amplifier 200. The semiconductor optical amplifier 200 includesa semiconductor element 210 and a current control unit 220. Thesemiconductor element 210 includes a first electrode 211 a, a secondelectrode 211 b, a P-type semiconductor layer 212 a, an N-typesemiconductor layer 212 b, and an active layer 213. The first electrode211 a and the second electrode 211 b are disposed opposite to eachother, and the P-type semiconductor layer 212 a, the N-typesemiconductor layer 212 b, and the active layer 213 are disposed betweenthem. The P-type semiconductor layer 212 a and the N-type semiconductorlayer 212 b are disposed opposite to each other, and the active layer213 is disposed between them. The first and second electrodes 211 a and211 b are connected with the current control unit 220.

In the case of applying the semiconductor optical amplifier 200 as thelight intensity changeable unit 15, current control unit 220 iscontrolled by the light intensity control unit 20. The seed light L_(S)output from the seed laser 14 is made incident on the active layer 213of the semiconductor element 210. The output light from thesemiconductor optical amplifier 200 is made incident on the amplifier 13via the dichroic mirror 16.

5.2.2 Operation

The current control unit 220 allows electric current to flow between thefirst electrode 211 a and the second electrode 211 b, according to acontrol signal input from the light intensity control unit 20. Whenelectric current flows between the first electrode 211 a and the secondelectrode 211 b, the active layer 213 is excited by the electriccurrent. In a state where the active layer 213 is excited, when the seedlight L_(S) is made incident on the active layer 213, the lightintensity of the seed light L_(S) is amplified.

This means that, when pulse-state electric current flows between thefirst electrode 211 a and the second electrode 211 b in a state wherethe seed light L_(S) that is CW laser light is applied to the activelayer 213, the seed light L_(S) can be output as the seed pulse lightL_(P). Specifically, the seed pulse light L_(P) can be generated whenthe first signal S1 is input from the light intensity control unit 20 tothe current control unit 220, and the suppression light L_(D) can begenerated when the second signal S2 is input.

As the semiconductor optical amplifier 200 does not depend onpolarization, there is no need to consider the polarization state of theseed light L_(S) like the case of applying the optical shutter 100 asthe light intensity changeable unit 15.

6. Exemplary Application of Solid-State Laser Device to Laser DeviceIncluding MO and Amplifier

Next, description will be given on an example in which a solid-statelaser device is used as an MO of a laser device for an exposure deviceincluding an MO and an amplifier, and an excimer laser is used as theamplifier.

6.1 Configuration

FIG. 18 schematically illustrates a configuration of the laser device 50for an exposure device including an MO and an amplifier. In FIG. 18, thelaser device 50 for an exposure device includes a solid-state lasersystem 51 as an MO, a first high reflective mirror 52, a second highreflective mirror 53, an amplifier 54, a laser control unit 55, and asynchronization control unit 56.

The laser control unit 55 is connected with the exposure device 4. Thelaser control unit 55 receives the oscillation preparation signal Rd anda first external trigger signal Tr1 from an exposure device control unit5 included in the exposure device 4. The laser control unit 55 transmitsthe oscillation preparation signal Rd received from the exposure devicecontrol unit 5, to the solid-state laser system 51 and the amplifier 54.The laser control unit 55 also transmits the first external triggersignal Tr1 received from the exposure device control unit 5, to thesynchronization control unit 56.

When the synchronization control unit 56 receives the first externaltrigger signal Tr1, the synchronization control unit 56 generates asecond external trigger signal Tr2 and a third external trigger signalTr3. The synchronization control unit 56 transmits the second externaltrigger signal Tr2 to the solid-state laser system 51, and transmits thethird external trigger signal Tr3 to the amplifier 54. Thesynchronization control unit 56 also controls the delay time of thethird external trigger signal Tr3 relative to the second externaltrigger signal Tr2. Specifically, the synchronization control unit 56controls the delay time such that electric discharge is performed in theamplifier 54 in synchronization with an input of the pulse laser light,output from the solid-state laser system 51, to the amplifier 54.

FIG. 19 schematically illustrates a configuration of the solid-statelaser system 51 illustrated in FIG. 18. In FIG. 19, the solid-statelaser system 51 includes a first solid-state laser device 60, a secondsolid-state laser device 61, a sum frequency wavelength conversion unit62, a high reflective mirror 63, a dichroic mirror 64, a synchronizationcircuit 65, and a solid-state laser control unit 66. The firstsolid-state laser device 60 and the second solid-state laser device 61have a configuration basically similar to that of the solid-state laserdevice 10 a according to the first embodiment or each of themodifications.

The first solid-state laser device 60 includes a first CW excitationlaser 70, a first amplifier 71, a first seed laser 72, a first lightintensity changeable unit 73, a first light intensity control unit 74,dichroic mirrors 75 and 76, and a wavelength conversion unit 77. Thefirst amplifier 71 includes a fiber amplifier 71 a and a solid-stateamplifier 71 b. The wavelength conversion unit 77 includes an LBOcrystal 77 a and a CLBO crystal 77 b.

The fiber amplifier 71 a includes an optical fiber made of Yb-dopedsynthetic quartz. The fiber amplifier 71 a may have multiple stages. Thesolid-state amplifier 71 b is a Yb-doped optical crystal.

The first CW excitation laser 70, the first seed laser 72, the firstlight intensity changeable unit 73, the first light intensity controlunit 74, and the dichroic mirrors 75 and 76 have configurations similarto those of the CW excitation laser 12, the seed laser 14, the lightintensity changeable unit 15, the light intensity control unit 20, andthe dichroic mirror 16 of the first embodiment or each of themodifications, respectively.

The first CW excitation laser 70 supplies first CW excitation light tothe first amplifier 71. The first CW excitation laser 70 includes a CWexcitation laser 70 a that supplies first CW excitation light to thefiber amplifier 71 a, and a CW excitation laser 70 b that supplies firstCW excitation light to the solid-state amplifier 71 b. The CW excitationlasers 70 a and 70 b are semiconductor lasers that output first CWexcitation light having a wavelength of about 976 nm.

The first seed laser 72 is a distributed-feedback semiconductor laserthat outputs CW laser light in a single longitudinal mode having awavelength of about 1,030 nm, as first seed light. It is preferable thatthe wavelength of the first seed light is in a wavelength range from1,020 nm to 1,090 nm.

The first CW excitation light output from the CW excitation laser 70 ais made incident on the fiber amplifier 71 a via the dichroic mirror 75.The first CW excitation light output from the CW excitation laser 70 bis made incident on the solid-state amplifier 71 b via the dichroicmirror 76.

The first CW excitation laser 70 and the first seed laser 72 areconnected with the solid-state laser control unit 66 via signal linesnot illustrated.

The second solid-state laser device 61 includes a second CW excitationlaser 80, a second amplifier 81, a second seed laser 82, a second lightintensity changeable unit 83, a second light intensity control unit 84,and a dichroic mirror 85. The second amplifier 81 is a fiber amplifierincluding an Er-doped quartz fiber. The second amplifier 81 may be afiber amplifier including a quartz fiber doped with both Er and Yb. Thefiber amplifier may have multiple stages.

The second CW excitation laser 80, the second seed laser 82, the secondlight intensity changeable unit 83, the second light intensity controlunit 84, and the dichroic mirror 85 have configurations similar to thoseof the CW excitation laser 12, the seed laser 14, the light intensitychangeable unit 15, the light intensity control unit 20, and thedichroic mirror 16 of the first embodiment or each of the modifications,respectively.

The second CW excitation laser 80 supplies second CW excitation light tothe second amplifier 81. The second CW excitation laser 80 is asemiconductor laser that outputs second CW excitation light having awavelength of about 976 nm. The second CW excitation light output fromthe second CW excitation laser 80 is made incident on the secondamplifier 81 via the dichroic mirror 85.

The second seed laser 82 is a distributed-feedback semiconductor laserthat outputs CW laser light in a single longitudinal mode having awavelength of about 1,554 nm, as second seed light. It is preferablethat the wavelength of the second seed light is in a wavelength rangefrom 1,550 nm to 1,555 nm.

The second CW excitation laser 80 and the second seed laser 82 areconnected with the solid-state laser control unit 66 via signal linesnot illustrated.

The synchronization circuit 65 receives the second external triggersignal Tr2 from the synchronization control unit 56 via the solid-statelaser control unit 66. Upon receipt of the second external triggersignal Tr2, the synchronization circuit 65 generates a fourth externaltrigger signal Tr4 and a fifth external trigger signal Tr5. Thesynchronization circuit 65 transmits the fourth external trigger signalTr4 to the first light intensity control unit 74, and transmits thefifth external trigger signal Tr5 to the second light intensity controlunit 84.

The first solid-state laser device 60 outputs first pulse laser light 67a having a wavelength of about 257.5 nm. The second solid-state laserdevice 61 outputs second pulse laser light 67 b having a wavelength ofabout 1,554 nm. The synchronization circuit 65 controls the timing ofthe fourth external trigger signal Tr4 and the fifth external triggersignal Tr5 such that the first pulse laser light 67 a and the secondpulse laser light 67 b are made incident on the sum frequency wavelengthconversion unit 62 almost simultaneously.

The high reflective mirror 63 is disposed such that the second pulselaser light output from the second solid-state laser device 61 isreflected and is made incident on the dichroic mirror 64. The dichroicmirror 64 is coated with a film that transmits, at a high rate, thefirst pulse laser light 67 a and reflects, at a high rate, the secondpulse laser light 67 b. The dichroic mirror 64 is disposed such that theoptical path axes of the first pulse laser light and the second pulselaser light match and the first pulse laser light 67 a and second pulselaser light 67 b are made incident on the sum frequency wavelengthconversion unit 62.

The sum frequency wavelength conversion unit 62 includes a first CLBOcrystal 62 a and a second CLBO crystal 62 b. The first CLBO crystal 62 aand the second CLBO crystal 62 b are disposed in this order on theoptical path of the first and second pulse laser light 67 a and 67 b.The sum frequency wavelength conversion unit 62 outputs third pulselaser light 67 c that is sum frequency light having a wavelength ofabout 193.4 nm.

The first and second high reflective mirrors 52 and 53 are disposed suchthat the third pulse laser light 67 c output from the sum frequencywavelength conversion unit 62 is made incident on the amplifier 54.

FIG. 20 schematically illustrates a configuration of the amplifier 54illustrated in FIG. 18. In FIG. 20, the amplifier 54 includes anamplifier control unit 90, a charging unit 91, a trigger correction unit92, a pulse power module (PPM) 94 including a switch 93, a chamber 95, apartial reflective mirror 96, and an output coupling mirror 97.

The chamber 95 is provided with windows 99 a and 99 b. The chamber 95 isfilled with laser gas including Ar gas, F₂ gas, and Ne gas, for example.In the chamber 95, a pair of discharge electrodes 98 is disposed. Thepair of discharge electrodes 98 is connected with an output terminal ofthe PPM 94.

In the amplifier 54, an optical resonator including a partial reflectivemirror 96 and an output coupling mirror 97 is configured. The partialreflective mirror 96 is configured such that a base plate made of a CaF₂crystal that transmits light having a wavelength of about 193.4 nm iscoated with a partial reflective film having a reflectance of 70% to90%. The output coupling mirror 37 is configured such that a base platemade of a CaF₂ crystal that transmits light having a wavelength of about193.4 nm is coated with a partial reflective film having a reflectanceof 10% to 20%.

The amplifier control unit 90 transmits the third external triggersignal Tr3 input from the synchronization control unit 56 to the triggercorrection unit 92. The trigger correction unit 92 corrects the timingof the third external trigger signal Tr3 and inputs it to the switch 93of the PPM 94 such that the pair of discharge electrodes 98 dischargeselectricity in synchronization with an input of the third pulse laserlight 67 c to the optical resonator.

6.2 Operation

When the laser control unit 55 receives the oscillation preparationsignal Rd from the exposure device control unit 5, the laser controlunit 55 transmits the oscillation preparation signal Rd to thesolid-state laser control unit 66 in the solid-state laser system 51 andto the amplifier control unit 90 in the amplifier 54.

When the solid-state laser control unit 66 receives the oscillationpreparation signal Rd, the solid-state laser control unit 66 allows thefirst and second seed lasers 72 and 82 to start laser oscillationoperation and allows the first and second CW excitation lasers 70 and 80to start laser oscillation operation. When the amplifier control unit 90receives the oscillation preparation signal Rd, the amplifier controlunit 90 allows preparation operation for laser amplification such asrotation of a fan, not illustrated, in the chamber 95.

Next, the solid-state laser control unit 66 outputs the set signal St tothe first and second light intensity control units 74 and 84. When thefirst and second light intensity control units 74 and 84 receive the setsignal St, the first and second light intensity control units 74 and 84input the second signal S2 to the first and second light intensitychangeable units 73 and 83, respectively. As a result, first suppressionlight and second suppression light are output from the first and secondlight intensity changeable units 73 and 83 to the first and secondamplifiers 71 and 81, respectively, whereby an increase of theamplification gain is suppressed.

Next, when the synchronization control unit 56 receives the firstexternal trigger signal Tr1 from the exposure device 4 via the lasercontrol unit 55, the synchronization control unit 56 generates thesecond external trigger signal Tr2 and the third external trigger signalTr3. The synchronization control unit 56 controls the delay time of thethird external trigger signal Tr3 relative to the second externaltrigger signal Tr2, and then, outputs the second external trigger signalTr2 to the solid-state laser control unit 66 and outputs the thirdexternal trigger signal Tr3 to the amplifier control unit 90.

Next, the solid-state laser control unit 66 outputs the second externaltrigger signal Tr2 to the synchronization circuit 65. When the secondexternal trigger signal Tr2 is input, the synchronization circuit 65generates the fourth external trigger signal Tr4 and the fifth externaltrigger signal Tr5. The synchronization circuit 65 controls the timingof the fourth external trigger signal Tr4 and the fifth external triggersignal Tr5, and then, outputs the fourth external trigger signal Tr4 tothe first light intensity control unit 74, and outputs the fifthexternal trigger signal Tr5 to the second light intensity control unit84.

When the fourth external trigger signal Tr4 is input, the first lightintensity control unit 74 outputs a ground signal to the first lightintensity changeable unit 73. Similarly, when the fifth external triggersignal Tr5 is input, the second light intensity control unit 84 outputsa ground signal to the second light intensity changeable unit 83. Whenthe grounds signals are input to the first and second light intensitychangeable units 73 and 84, transmission of the first seed light and thesecond seed light is suppressed in the first and second light intensitychangeable units 73 and 84. Thereby, output of the first suppressionlight and the second suppression light is stopped, whereby theamplification gain is increased in the first and second amplifiers 71and 81.

The first light intensity control unit 74 outputs the first signal S1 tothe first light intensity changeable unit 73 after the certain time Tdelapsed from the input of the fourth external trigger signal Tr4.Similarly, the second light intensity control unit 84 outputs the firstsignal S1 to the first light intensity changeable unit 73 after thecertain time Td elapsed from the input of the fifth external triggersignal Tr5. When the first signal S1 is input to the first lightintensity changeable unit 73, the first light intensity changeable unit73 outputs first seed pulse light in which the first seed light ispulsed, to the first amplifier 71. Similarly, when the first signal S1is input to the second light intensity changeable unit 83, the secondlight intensity changeable unit 83 outputs second seed pulse light inwhich the second seed light is pulsed, to the second amplifier 81.

The first seed pulse light input to the first amplifier 71 is amplifiedin the first amplifier 71, and is output to the wavelength conversionunit 77 as first amplified light having a wavelength of about 1,030 nm.The first amplified light input to the wavelength conversion unit 77 isconverted to fourth harmonic light having a wavelength of about 257.5 nmby the LBO crystal 77 a and the CLBO crystal 77 b. The fourth harmoniclight is output from the first solid-state laser device 60 as the firstpulse laser light 67 a.

Meanwhile, the second seed pulse light input to the second amplifier 81is amplified in the second amplifier 81, and is output as secondamplified light having a wavelength of about 1,554 nm. The secondamplified light is output from the second solid-state laser device 61 asthe second pulse laser light 67 b.

The first pulse laser light 67 a output from the first solid-state laserdevice 60 and the second pulse laser light 67 b output from the secondsolid-state laser device 61 are made incident on the sum frequencywavelength conversion unit 62 almost simultaneously. The first pulselaser light 67 a having the wavelength of about 257.5 nm and the secondpulse laser light 67 b having the wavelength of about 1,554 nm overlapwith each other on the first CLBO crystal 62 a included in the sumfrequency wavelength conversion unit 62.

In the CLBO crystal 62 a, pulse laser light having a wavelength of about220.9 nm, corresponding to the sum frequency of the wavelength of about257.5 nm and the wavelength of about 1,554 nm, is generated. Then, inthe second CLBO crystal 62 b, third pulse laser light 67 c having awavelength of about 193.4 nm, corresponding to the sum frequency of thewavelength of about 220.9 nm and the wavelength of about 1,554 nm, isgenerated. The third pulse laser light 67 c is output from thesolid-state laser system 51, and is made incident on the partialreflective mirror 96 of the amplifier 54 via the first and second highreflective mirrors 52 and 53.

The third pulse laser light 67 c is input as seed light into the opticalresonator of the amplifier 54 including the partial reflective mirror 96and the output coupling mirror 97. In synchronization with such aninput, in the chamber 95 of the amplifier 54, inverted population isformed by discharging between the pair of discharge electrodes 39. Atthat time, the trigger correction unit 92 corrects the timing of thethird external trigger signal Tr3 and inputs it to the switch 93 of thePPM 94 such that the third pulse laser light 67 c is amplifiedefficiently in the amplifier 54. As a result, the optical resonator ofthe amplifier 54 performs amplification and oscillation, and theamplified pulse laser light is output from the output coupling mirror97. The amplified pulse laser light has a wavelength of about 193.4 nm,and is input to the exposure device 4.

The first light intensity control unit 74 outputs the second signal S2to the first light intensity changeable unit 73 during the time afterthe first signal S1 is output to the first light intensity changeableunit 73 until the fourth external trigger signal Tr4 is input.Similarly, the second light intensity control unit 84 outputs the secondsignal S2 to the second light intensity changeable unit 83 during thetime after the first signal S1 is output to the second light intensitychangeable unit 83 until the fifth external trigger signal Tr5 is input.

Thereby, during the time after the third pulse laser light 67 c isoutput from the solid-state laser system 51 until the second externaltrigger signal Tr2 is input to the solid-state laser system 51, anincrease of the amplification gain in the first and second amplifiers 71and 81 is suppressed. The second signal S2 may have a signal waveformillustrated in any of FIGS. 8D, 11D, 13D, and 14D.

6.3 Effect

As described above, the first and second light intensity control units74 and 84 in the solid-state laser system 51 respectively output thefirst signal S1 and the second signal S2 to the first and second lightintensity changeable units 73 and 83 in synchronization with inputs ofthe external trigger signals. Accordingly, the light intensity and thepulse energy of the third pulse laser light 67 c output from thesolid-state laser system 51 are constant regardless of the pulseinterval of the external trigger signals.

Further, in the solid-state laser system 51, an increase of the lightintensity and the pulse energy of the third pulse laser light 67 c atthe head of the burst output, immediately after the start of the burstperiod after the pause period in the burst operation, is suppressed. Asa result, variations in the light intensity and the pulse energy of thepulse laser light amplified by the amplifier 54 are also suppressed.

Even in the solid-state laser system 51, the certain time Td that is adelay time of a signal by the delay circuits included in the first andsecond light intensity control units 74 and 84 is a fixed value ratherthan a value that varies according to the pulse interval T of theexternal trigger signals Tr. The fixed value may be set to each of thedelay circuits, included in the first and second light intensity controlunits 74 and 84, by the solid-state laser control unit 66, according tothe pulse energy of the third pulse laser light 67 c output from thesolid-state laser system 51. For example, the solid-state laser controlunit 66 sets the fixed value to be a larger value as the required pulseenergy of the third pulse laser light 67 c output from the solid-statelaser system 51 is higher.

6.4 Definition of Wavelength Conversion Threshold

In the solid-state laser system 51, it is preferable that the wavelengthconversion threshold is defined to include the wavelength conversionefficiency of the sum frequency wavelength conversion unit 62, inaddition to the wavelength conversion efficiency of the wavelengthconversion unit 77. This means that it is preferable to define thewavelength conversion threshold while considering up to the final stageof the wavelength conversion. Specifically, as for the first solid-statelaser device 60, a wavelength conversion threshold obtained by combiningthe wavelength conversion efficiencies of both the wavelength conversionunit 77 and the sum frequency wavelength conversion unit 62 is used as afirst wavelength conversion threshold. As for the second solid-statelaser device 61, the wavelength conversion threshold of the sumfrequency wavelength conversion unit 62 is used as a second wavelengthconversion threshold.

The first wavelength conversion threshold is defined as a lightintensity of incident light in which a first wavelength conversionefficiency takes a given value. The first wavelength conversionefficiency is a wavelength conversion efficiency in which incident lightfrom the first amplifier 71 to the wavelength conversion unit 77 isconverted into sum frequency light by the wavelength conversion unit 77and the sum frequency wavelength conversion unit 62. The secondwavelength conversion threshold is defined as a light intensity ofincident light in which a second wavelength conversion efficiency takesa given value. The second wavelength conversion efficiency is awavelength conversion efficiency in which incident light from the secondamplifier 81 to the sum frequency wavelength conversion unit 62 isconverted into sum frequency light by the sum frequency wavelengthconversion unit 62. It is preferable that the given value is in a rangefrom 1% to 2%, for example. It is also preferable that the given valueis in a range from 0% to 0.01%.

Further, the first wavelength conversion threshold may be defined basedon a ratio of a second light intensity to a first light intensity. Here,the first light intensity is a light intensity of sum frequency lightoutput from the solid-state laser system 51 according to the firstsignal S1 generated in the first solid-state laser device 60, and thesecond light intensity is a light intensity of sum frequency lightgenerated by the solid-state laser system 51 according to the secondsignal S2 generated in the first solid-state laser device 60. Similarly,the second wavelength conversion threshold may be defined based on aratio of a fourth light intensity to a third light intensity. Here, thethird light intensity is a light intensity of sum frequency light outputfrom the solid-state laser system 51 according to the first signal S1generated in the second solid-state laser device 61, and the fourthlight intensity is a light intensity of sum frequency light generated bythe solid-state laser system 51 according to the second signal S2generated in the second solid-state laser device 61. For example, thefirst wavelength conversion threshold may be defined as a value in whichthe ratio of the second light intensity to the first light intensity isin a range from 0% to 10%. Further, the second wavelength conversionthreshold may be defined as a value in which the ratio of the fourthlight intensity to the third light intensity is in a range from 0% to10%.

The light intensity of first secondary light generated by an input offirst suppression light to the first amplifier 71 is less than the firstwavelength conversion threshold. The light intensity of second secondarylight generated by an input of second suppression light to the secondamplifier 81 is less than the second wavelength conversion threshold.

6.5 Modification Related to Wavelength Conversion Unit

In the solid-state laser system 51 illustrated in FIG. 19, thewavelength conversion unit 77 is provided in the first solid-state laserdevice 60. However, the wavelength conversion unit 77 may be eliminated.This means that it is possible to have a configuration in which firstpulse laser light output from the first solid-state laser device 60 notincluding a wavelength conversion unit and second pulse laser lightoutput from the second solid-state laser device 61 not including awavelength conversion unit are made incident on the sum frequencywavelength conversion unit 62.

In that case, the first wavelength conversion threshold of the sumfrequency wavelength conversion unit 62 is expressed by a ratio of thesecond light intensity to the first light intensity. The first lightintensity is the light intensity of the sum frequency light generated byan input of the first amplified light to the sum frequency wavelengthconversion unit 62, and the second light intensity is the lightintensity of the sum frequency light generated by an input of the firstsecondary light to the sum frequency wavelength conversion unit 62.Similarly, the second wavelength conversion threshold of the sumfrequency wavelength conversion unit 62 is expressed by a ratio of thefourth light intensity to the third light intensity. The third lightintensity is the light intensity of the sum frequency light generated byan input of the second amplified light to the sum frequency wavelengthconversion unit 62, and the fourth light intensity is the lightintensity of the sum frequency light generated by an input of the secondsecondary light to the sum frequency wavelength conversion unit 62. Forexample, the first wavelength conversion threshold may be defined as avalue in which the ratio of the second light intensity to the firstlight intensity is in a range from 0% to 10%. Further, the secondwavelength conversion threshold may be defined as a value in which theratio of the fourth light intensity to the third light intensity is in arange from 0% to 10%.

6.6 Modifications of Amplifier

The amplifier 54 illustrated in FIG. 20 is applied to the laser device50 for the exposure device. However, the amplifiers of variousconfigurations may be applicable.

6.6.1 First Modification

FIG. 21 schematically illustrates a configuration of an amplifier 300according to a first modification. In FIG. 21, the amplifier 300includes a concave mirror 310 and a convex mirror 320, instead of thepartial reflective mirror 96 and the output coupling mirror 97 in theconfiguration of the amplifier 54 illustrated in FIG. 20. The concavemirror 310 and the convex mirror 320 are disposed such that the thirdpulse laser light 67 c passes through the discharge space between thepair of discharge electrodes 98 three times whereby the beam isexpanded. The other configurations of the amplifier 300 are similar tothose of the amplifier 54.

In the amplifier 300, the third pulse laser light 67 c made incident onthe amplifier 300 is reflected by the concave mirror 310 and the convexmirror 320 to thereby pass through the discharge space between the pairof discharge electrodes 98 three times. Thereby, the beam of the thirdpulse laser light 67 c is expanded and amplified, and is output towardthe exposure device 4.

6.6.2 Second Modification

FIG. 22 schematically illustrates a configuration of an amplifier 400according to a second modification. In FIG. 22, the amplifier 400includes the chamber 95, an output coupling mirror 410, and highreflective mirrors 420 to 422. The amplifier 400 also includes theamplifier control unit 90, the charging unit 91, the trigger correctionunit 92, and the pulse power module 94 including the switch 93, althoughnot illustrated, similar to the amplifier 54 illustrated in FIG. 20. Theamplifier 400 may also include a high reflective mirror that leads thethird pulse laser light 67 c from the solid-state laser system 51 to theamplifier 400, and a high reflective mirror that leads the pulse laserlight output from the amplifier 400 to the exposure device 4.

The chamber 95 may be provided with the windows 99 a and 99 b. In thechamber 95, a pair of discharge electrodes 98 is disposed. The pair ofdischarge electrodes 98 may be disposed opposite to a directionorthogonal to the sheet surface in FIG. 22. The output coupling mirror410 and the high reflective mirrors 420 to 422 constitute an opticalresonator. In the amplifier 400, the third pulse laser light 67 crepeatedly travels through the output coupling mirror 410, the highreflective mirror 420, the discharge space between the pair of dischargeelectrodes 98, the high reflective mirror 421, the high reflectivemirror 422, and the discharge space between the pair of dischargeelectrodes 98, in this order, and is amplified.

7. Other Modifications

In the solid-state laser device described above, a CW excitation laseris used as a semiconductor laser that outputs CW laser light having awavelength of about 976 nm. However, the semiconductor laser may bechanged according to the type of an amplifier that supplies CWexcitation light. For example, with respect to an Yb-doped fiberamplifier, it is preferable to use a semiconductor laser that outputs CWlaser light having a wavelength of about 976 nm as a CW excitationlaser. However, as another example, it is possible to use asemiconductor laser that outputs CW laser light having a wavelength ofabout 915 nm or about 969 nm. Further, with respect to an Yb-doped fiberamplifier, it is preferable to use a semiconductor laser that outputs CWlaser light having a wavelength of about 938 nm as a CW excitationlaser.

In the solid-state laser device described above, CW excitation lightoutput from a CW excitation laser is made incident on an amplifier via adichroic mirror. When the amplifier is a fiber amplifier, it is possibleto use a pump combiner instead of the dichroic mirror.

In the solid-state laser device described above, the wavelengthconversion unit includes a LBO crystal and a CLBO crystal, and isconfigured to generate fourth harmonic light. However, variousmodifications can be made in the configuration of the wavelengthconversion unit. It is only necessary that the wavelength conversionunit includes at least one of an LBO crystal, a BBO crystal, a CLBOcrystal, and a KBBF (KBe₂BO₃F₂) crystal, and is configured to generatesecondary or higher harmonic light.

Further, in the laser device for an exposure device described above, thesum frequency wavelength conversion unit includes two CLBO crystals.However, various modifications can be made to the configuration of thesum frequency wavelength conversion unit. It is only necessary that thesum frequency wavelength conversion unit includes at least one CLBOcrystal, and is configured to generate third pulse laser light having asum frequency of first pulse laser light and second pulse laser light.

The description provided above is intended to provide just exampleswithout any limitations. Accordingly, it will be obvious to thoseskilled in the art that changes can be made to the embodiments of thepresent disclosure without departing from the scope of the accompanyingclaims.

The terms used in the present description and in the entire scope of theaccompanying claims should be construed as terms “without limitations”.For example, a term “including” or “included” should be construed as“not limited to that described to be included”. A term “have” should beconstrued as “not limited to that described to be held”. Moreover, anindefinite article “a/an” described in the present description and inthe accompanying claims should be construed to mean “at least one” or“one or more”.

What is claimed is:
 1. A solid state laser device comprising: a seedlaser configured to output seed light that is continuous wave laserlight; a light intensity changeable unit configured to change a lightintensity of the seed light to make the seed light pulsed and output thepulsed seed light as seed pulse light; a CW excitation laser configuredto output continuous wave excitation light; an amplifier configured toamplify the seed pulse light and output the amplified seed pulse lightas amplified light, based on an amplification gain increased by lightexcitation by the continuous wave excitation light; a wavelengthconversion unit configured to convert a wavelength of the amplifiedlight and output harmonic light; and a light intensity control unitconfigured to control the light intensity changeable unit according toan input of an external trigger signal, the light intensity control unitallowing the light intensity changeable unit to output the seed pulselight after a certain time elapsed from an input of the external triggersignal each time the external trigger signal is input, and allowing thelight intensity changeable unit to output suppression light thatsuppresses an increase of the amplification gain in a period after anoutput of the seed pulse light until an input of a next external triggersignal.
 2. The solid-state laser device according to claim 1, whereinthe certain time takes a fixed value.
 3. The solid-state laser deviceaccording to claim 2, wherein the fixed value is set to the lightintensity control unit according to pulse energy of the harmonic light.4. The solid-state laser device according to claim 1, wherein the lightintensity control unit allows the light intensity changeable unit tosuppress transmission of the seed light such that the amplification gainis increased during a period from an input of the external triggersignal until the certain time elapses.
 5. The solid-state laser deviceaccording to claim 4, wherein a light intensity of secondary lightgenerated by an input of the suppression light to the amplifier is lessthan a wavelength conversion threshold of the wavelength conversionunit.
 6. The solid-state laser device according to claim 5, wherein thewavelength conversion threshold represents a light intensity in which awavelength conversion efficiency of the wavelength conversion unit is ina range from 1% to 2%.
 7. The solid-state laser device according toclaim 5, wherein the wavelength conversion threshold represents a lightintensity in which a wavelength conversion efficiency of the wavelengthconversion unit is in a range from 0% to 0.01%.
 8. The solid-state laserdevice according to claim 5, wherein the wavelength conversion thresholdis expressed by a ratio of a second light intensity to a first lightintensity, the first light intensity being a light intensity of harmoniclight generated by wavelength conversion of the amplified light by thewavelength conversion unit, the second light intensity being a lightintensity of harmonic light generated by wavelength conversion of thesecondary light by the wavelength conversion unit, and the wavelengthconversion threshold is defined as a light intensity in which the ratioof the second light intensity to the first light intensity is in a rangefrom 0% to 10%.
 9. The solid-state laser device according to claim 1,wherein the suppression light is continuous wave laser light or pulselaser light.
 10. The solid-state laser device according to claim 9,wherein the light intensity control unit gradually increases a lightintensity of the suppression light during a certain period after theseed pulse light is output.
 11. The solid-state laser device accordingto claim 1, wherein the light intensity changeable unit is an opticalshutter or a semiconductor optical amplifier.
 12. The solid-state laserdevice according to claim 1, wherein the amplifier includes a Yb-dopedsolid-state amplifier and/or a Yb-doped fiber amplifier.
 13. Thesolid-state laser device according to claim 1, wherein the wavelengthconversion unit includes at least one of an LBO (LiB₃O₅) crystal, a BBO(β-BaB₂O₄) crystal, a CLBO (CsLiB₆O₁₀) crystal, and a KBBF (KBe₂BO₃F₂)crystal, and generates secondary or higher harmonic light.
 14. A solidstate laser system comprising: a first solid-state laser deviceincluding: a first seed laser configured to output first seed light thatis continuous wave laser light; a first light intensity changeable unitconfigured to change a light intensity of the first seed light to makethe first seed light pulsed and output the pulsed first seed light asfirst seed pulse light; a first CW excitation laser configured to outputfirst continuous wave excitation light; a first amplifier configured toamplify the first seed pulse light and output the amplified first seedpulse light as first amplified light, based on an amplification gainincreased by light excitation by the first continuous wave excitationlight; and a first light intensity control unit configured to controlthe first light intensity changeable unit according to an input of anexternal trigger signal, the first light intensity control unit allowingthe first light intensity changeable unit to output the first seed pulselight after a certain time elapsed from an input of the external triggersignal each time the external trigger signal is input, and allowing thefirst light intensity changeable unit to output first suppression lightthat suppresses an increase of the amplification gain of the firstamplifier in a period after an output of the first seed pulse lightuntil an input of a next external trigger signal, a second solid-statelaser device including: a second seed laser configured to output secondseed light that is continuous wave laser light; a second light intensitychangeable unit configured to change a light intensity of the secondseed light to make the second seed light pulsed and output the pulsedsecond seed light as second seed pulse light; a second CW excitationlaser configured to output second continuous wave excitation light; asecond amplifier configured to amplify the second seed pulse light andoutput the amplified second seed pulse light as second amplified light,based on an amplification gain increased by light excitation by thesecond continuous wave excitation light; and a second light intensitycontrol unit configured to control the second light intensity changeableunit according to an input of an external trigger signal, the secondlight intensity control unit allowing the second light intensitychangeable unit to output the second seed pulse light after a certaintime elapsed from an input of the external trigger signal each time theexternal trigger signal is input, and allowing the second lightintensity changeable unit to output second suppression light thatsuppresses an increase of the amplification gain of the second amplifierin a period after an output of the second seed pulse light until aninput of a next external trigger signal, and a sum frequency wavelengthconversion unit configured to generate third pulse laser light includinga sum frequency of the first pulse laser light output from the firstsolid-state laser device and the second pulse laser light output fromthe second solid-state laser device.
 15. The solid-state laser systemaccording to claim 14, wherein the certain time takes a fixed value. 16.The solid-state laser system according to claim 15, wherein the fixedvalue is set to the first light intensity control unit and the secondlight intensity control unit according to pulse energy of the thirdpulse laser light.
 17. The solid-state laser system according to claim14, wherein the first light intensity control unit allows the firstlight intensity changeable unit to suppress transmission of the firstseed light such that the amplification gain of the first amplifier isincreased during a period from an input of the external trigger signaluntil the certain time elapses, and the second light intensity controlunit allows the second light intensity changeable unit to suppresstransmission of the second seed light such that the amplification gainof the second amplifier is increased during a period from an input ofthe external trigger signal until the certain time elapses.
 18. Thesolid-state laser system according to claim 17, wherein a lightintensity of first secondary light generated by an input of the firstsuppression light to the first amplifier is less than a first wavelengthconversion threshold of the sum frequency wavelength conversion unit,and a light intensity of second secondary light generated by an input ofthe second suppression light to the second amplifier is less than asecond wavelength conversion threshold of the sum frequency wavelengthconversion unit.
 19. The solid-state laser system according to claim 18,wherein the first wavelength conversion threshold is expressed by aratio of a second light intensity to a first light intensity, the firstlight intensity being a light intensity of sum frequency light generatedby an input of the first amplified light to the sum frequency wavelengthconversion unit, the second light intensity being a light intensity ofsum frequency light generated by an input of the first secondary lightto the sum frequency wavelength conversion unit, the first wavelengthconversion threshold is defined as a light intensity in which the ratioof the second light intensity to the first light intensity is in a rangefrom 0% to 10%, the second wavelength conversion threshold is expressedby a ratio of a fourth light intensity to a third light intensity, thethird light intensity being a light intensity of sum frequency lightgenerated by an input of the second amplified light to the sum frequencywavelength conversion unit, the fourth light intensity being a lightintensity of sum frequency light generated by an input of the secondsecondary light to the sum frequency wavelength conversion unit, and thesecond wavelength conversion threshold is defined as a light intensityin which the ratio of the fourth light intensity to the third lightintensity is in a range from 0% to 10%.
 20. The solid-state laser systemaccording to claim 17, wherein the first solid-state laser deviceincludes a wavelength conversion unit configured to convert thewavelength of the first amplified light to generate harmonic light, andoutput the harmonic light as the first pulse laser light.
 21. Thesolid-state laser system according to claim 20, wherein the wavelengthconversion unit includes at least one of an LBO (LiB₃O₅) crystal, a BBO(β-BaB₂O₄) crystal, a CLBO (CsLiB₆O₁₀) crystal, and a KBBF (KBe₂BO₃F₂)crystal, and the sum frequency wavelength conversion unit includes atleast one CLBO crystal.
 22. The solid-state laser system according toclaim 14, wherein each of the first light intensity changeable unit andthe second light intensity changeable unit includes an optical shutteror a semiconductor optical amplifier.
 23. The solid-state laser systemaccording to claim 14, wherein the first amplifier includes a Yb-dopedsolid-state amplifier and/or a Yb-doped fiber amplifier, and the secondamplifier includes an Er-doped fiber amplifier.
 24. A laser device foran exposure device comprising: the solid-state laser system according toclaim 14; and an amplifier including an excimer laser device configuredto amplify the third pulse laser light output from the solid-state lasersystem.